1. Field of the Invention
The invention relates to the remote measurement of a physical magnitude of a tire while a vehicle is running, particularly through a radio frequency link and a passive component.
More particularly, the invention relates to resonant structures with bulk waves within piezoelectric films that are adapted for this restrictive use. The hybrid structure according to the invention may be optimized to increase its quality factor and its coupling factor in its frequency range.
The invention also relates to the use of hybrid resonators in a vehicle ground connection, for example at the tire, to make a remote wireless measurement of the running parameters such as the temperature of the rubber of a tire or the internal air pressure. The invention also relates to the use of resonant structures to transform active MEMS (micro-electromechanical systems) to passive MEMS, in other words that can be remotely queried and that are not connected to a power supply source.
2. Description of Related Art
To increase automobile safety, it appears desirable to integrate systems for real time monitoring of the performances of tires and/or their usage conditions. To achieve this, a tire instrumentation channel is aimed at integrating electronic devices such as sensors, for example so as to monitor parameters related to the use and/or wear of tires. For example, marketed TPMSs (“Tire Pressure Monitoring Systems”), that usually consist of sensors integrated into tire valves, notify the driver if his tire pressure is correct and detect leaks.
As soon as it becomes possible to consider measuring a physical parameter of a tire mounted on its wheel under running conditions, the problem of the energy available to make this measurement and transmit it to a control system inside or outside the vehicle arises; electronic devices must also comprise a means of supplying power to sensors and retrieving and even processing signals. Existing solutions used for most currently installed TPMS systems are based on the use of batteries associated with strategies for energy management so that they do not need to be replaced throughout the life of the tires.
However, the ideal solution relates to passive sensors, in other words sensors which do not require any energy source onboard the tire/wheel assembly but are powered by a remote radio frequency wave or by an auto-energy generation system associated with the tire. In the case of a power supply through a radio frequency wave, a query signal is sent to the sensor on which an antenna is installed; after the signal has been received, the sensor sends a radio wave that contains information about the physical parameter to which it is directly or indirectly sensitive.
Thus, it is known for example in document EP 0 937 615, that Surface Acoustic Wave (SAW) sensors can be used to passively measure physical parameters such as bond of the tire by radio frequency waves; this SAW sensors solution was developed particularly for data transmission (US 2005/093688).
SAW sensors may be of the “delay line” type (the phase difference between several echoes generated by the sensor will depend on the parameter to be measured) or the “resonator” type (the resonant frequency of the sensor will depend on the parameter to be measured). Resonator type sensors, due to their compact size, are usually better adapted to the measurement of a physical parameter of the tire if access to this parameter requires integration of the sensor during manufacturing of the tire.
However, a high performance measurement by a resonator type sensor requires a high resonance quality for optimum detection precision of its resonant frequency, and the lowest possible insertion losses in the resonator (to optimize the use of energy sent by the query radio wave since the system is passive), and a sufficient sensitivity to the physical parameter to be measured for the envisaged application. The SAW resonator may be limited for the envisaged applications, in order to optimize these three performance criteria without increasing the final size of the sensor; SAW resonators with minimum insertion losses and therefore with maximum coupling (10% instead of 1%, for example by varying the piezoelectric material of the support) have insufficient resonance quality factors.
Furthermore, the sensitivity of SAW resonators to the physical parameter to be measured, and particularly the temperature, may be much too high to guarantee that radio emission standards (FCC or ETSI) are respected, particularly in the 433.92 MHz ISM band: thus the high thermal sensitivity causes resonance outside the authorized frequency bands. Finally, such resonant surface wave structures must be a certain size related to the acoustic wave length and their very configuration, that requires a minimum length to perform their spectral function: a typical sensor is usually 5 mm×5 mm.
One envisaged alternative could be the use of bulk wave resonators based on putting a piezoelectric material blade into vibration, in which two facing electrodes clamp a plate of piezoelectric material; application of a radio frequency field to the dipole terminals thus created, generates deformation of the plate by inverse piezoelectric effect, depending on the couplings allowed by the crystalline orientation of its component material.
Quartz is found to be the preferred material for this type of application, taking account of its thermoelastic properties (high mechanical quality coefficients, existence of orientations compensated for temperature effects, etc.). Such a typical resonator operates at the order of 1 MHz, which is much too low for optimum radio frequency detection; in order to increase the frequency, it becomes necessary to thin the usual solid materials down to the thicknesses that make any industrial application (for which the minimum thickness of the plates is of the order of 30 μm) risky: a frequency of 1 GHz represents the practical limit for the use of classical bulk wave resonators.
These bulk acoustic wave BAW oscillators cannot be used in practice, particularly for the measurement of running parameters in a severe environment: no alternative to SAW sensors has yet been marketed.